It is important to determine the wettability of rocks with the water and oil that may be continued therein. For this purpose, the rock must be drained, namely fluids must be displaced to decrease the water saturation, then soaked, with the latter term signifying displacement of fluids allowing the water saturation (Sw) of the rock to be increased. The capillary pressure at a point is defined as the difference Pc in the equilibrium between the oil pressure Po and the water pressure Pwo This parameter makes sense only if the two fluids are in continuous phase in the porous medium. For a water-wettable medium, only positive values have a meaning. When the medium has mixed wettability, however, the fluids may remain in continuous phase for both positive and for negative capillary pressures (Pc).
For an application of this type, a complete capillary pressure measuring cycle must hence comprise (FIG. 1):
a) positive primary drainage of a sample initially 100% saturated with water (Curve 1);
b) positive soaking (curve 2);
c) negative soaking (curve 3);
d) negative drainage (curve 4); and
e) positive secondary drainage (curve 5).
Knowledge of various parameters, particularly the wettability of the rocks, is useful particular when assisted recovery of a formation is to be carried out, draining the effluents it contains by injecting a fluid under pressure, after determining the fluid (water or gas) that is most appropriate for displacing the effluents.
The invention also has civil engineering applications for hydrology studies of the ground to determine its degree of pollution for example, and in building for testing construction materials in order in particular to make decisions on water-repellant treatments for example.
Measurement of the capillary pressure of rocks saturated with fluids by subjecting them to centrifugation at a progressive speed and measuring the quantity of fluid produced as a function of speed is known. The liquid-saturated sample is placed in an enclosure whose axis is in the direction of the centrifugal force and another fluid is injected to take the place of the expelled fluid as it is expelled. In the re-soaking phase, the speed is decreased in order to study the return of the original fluid to the sample. The pressure field created by centrifugation is expressed as a function of density r, radius R, and angular velocity w, y the relation: 1/2w.sup.2..GAMMA.(Rmax.sup.2 --R.sup.2), for each fluid. The pressure of the two fluids at the sample outlet must be the same and must become zero at the outlet. Local saturations with this type of method are calculated by a negation program from the total quantity of water expelled from the sample. This method is implemented for example in patent applications FR-A-2,666,147 and EN.92/15215 by the applicant.
Another method, known as the "dynamic" method, consists of placing a sample in an elongate enclosure terminating at both its ends in water-permeable membranes. At one end, oil is injected under pressure into the enclosure. Water is also injected, but this injection is effected through the membrane and at a lower pressure. At the opposite end, the oil is evacuated directionally while the water leaves through the end membrane. By adjusting the oil and water injection rates, the capillary pressure is made to be the same at the inlet to the enclosure as at its outlet, which brings about uniform saturation that can be deduced from the fluid balance. The capillary pressure is obtained for example by measuring the difference between the oil and water pressures at the outlet of the enclosure. Such a method is described in particular by H. W. Brown in "Capillary Pressure Investigations," Petroleum Transactions AIME, Vol. 192, 1951.
U.S. Pat. No. 4,924,187 teaches a method for making wettability measurements on samples of porous rock in a confinement cell. The bar, of which the sample to be studied is composed, is placed inside an elongate deformable sheath associated with pressure means for applying a confinement pressure thereon laterally. At one end, the cell is closed by a porous membrane permeable to a first fluid such as brine saturating the bar but not to a second fluid such as oil or a gas. The porous membrane is made of a ceramic plate. This displacement fluid is injected under pressure at the opposite end of the cell and the first fluid, driven out of the sample, is collected at the first end. Between the conducting side pieces closing the cell at its two ends, an electrical voltage is applied. By means of electrodes that radially traverse the sheath and in contact with the sample at different locations along its length, the interelectrode electrical resistance and its variations are measured as the wetting fluid is drained from the bar.